Current limiting fuse



Jan. 14, 1.9.47. Q Gl? sun-5 2,414,344

CURRENT LIMITING FUSE Filed NOV. 1l, 1942 Inventor* Chauncey G. Suit 5 b/VWTMH tdHis-:y Attorneg.

'Patented Jan. 14, 19.47

CURRENT LIMITING FSE .Chauncey G. Suits, Schenectady, N. Y., assigner toGeneral Electric Company, a corporation of New York Application November1,1, 1942, serial No. 465,206

y 7 Claims.

My invention relates to fuses and more particularly to fuses ofthecurrent limiting type.

Current limiting fuses arevknown'which con# sist in the main of afusible Aelement in the form of one or more cnductorsof suitable metalsuch 5 as silver or zinc embedded in an inert granular or refractoryarc-extinguishing material of high dielectric strength such` as sand, orrelatively finely divided quartz, the conductor or conductors in someinstances being wound on a supporting core of high temperature resistantceramic material.I To these parts are added an enclosing housing andterminals for the conductors.

Upon subjection of such fuse to currents of large magnitude, suchasfuponshort circuit, the 15 fusible element attains fusing temperature anddissipation only at a point of reduced crosssec tion, whereupon arcingoccurs and the metal va 2o pors rapidly expand to many timesy the volumeoriginally occupied by the fusible element itself. The force of thisexpansion throws the metal vapors outof the arc path and into the spacesbetween the granules of inert ller material.

These vapors condense upon the surfaces of the relatively cool granulesand no longer are available for current conduction so that arcingis'conf lined to the small space previously occupied by the fusibleelement. The intimate' physical con- 30 tact between the hot arc columnand the relatively cool granules causes an exceedingly rapid transfer ofheat from the arc to the granules res sulting in a very high voltagegradient or a high voltage drop per unit length of arc column. The

form a hollow shell or fulguritel consisting 01.4.0 fused quartz if theAarc-extinguishing material is granular quartz, and a small percentageof metal from which the fusible conductor is made.l This fused shell hasthe properties of an insulator and in' the normal operation of the fusedoesnot con- 45 stltute a currenteconducting element in the currentpath.

As` a result of the actions referred toin` the preceding paragraph thereis in eect suddenly inserted into the path of the current at the in-V 5ostant of melting of the fusible element, the high l resistance formed bythe arc column which initially limits the current to a magnitude whichis only a small fraction of the current lwhich the associated is capableof producing. The inserted resistance causes rapid current decay andsubsequent interruption of the current flow with negligible mechanicaldisturbance,there being no gassing or discharge of vapors or particlesand practically no noise. Y

I have discovered that if, instead of an arc quenching JmeansAcomprising an inert granular material, an arc quenching meanscomprising a pair of flat-spaced surfaces of insulating material such asfused quartz or the like, is used around the fusible element sothat thefusible element is mounted between the plates, the current limitingaction of the fuse is increased by a relatively large factor. It is mybelief that the reason fog` this higher interrupting ability isdissipates or vaporizes throughout a fusible section of considerablelength as distinguished from based upon the following theory. It shouldbe understood that this theory is presented by way of explanation onlyand I do not desire to be bound by this theory in the event that it isnot the true explanation for the greater interruptingv ability ofcurrent limitingy fuses embodying my invention. It is believed that thespace between the flat surfaces forms a reservoir for the hot gasesdeveloped by the arc whereby a large amount of the heat developed isremoved by conduction and convection from the immediate vicinitytofthearc. This action is believed to produce a very satisfactory currentlimit/ingl action.

It is an object of my invention toprovide a new and improvedcurrentvlimiting fuse having a greater current interrupting ability thancurgrent limiting fusesA known heretofore. Further objects andvadvantages of my inven-ll tion will become apparent as the following description proceeds and the features of novelty A which-characterize myinvention will be pointed Y out with particularity in the claims annexedto and forming a part of this specification.

Referring, now to the drawing, Fig. 1 is a crosssectlonal view of acurrent limiting fuse embodying my invention in which the fusibleelement, for the purpose of simplifying the disclosure, has been shownas a straight conductor extending from end to end ofthe fuse enclosureand Figs. 2 and 3 are partial views of modifications embodying'th'eteachingsk of my invention.

'I'he current limiting fuse of Fig. 1 is illustrated as comprising anenclosing Vtubular casing lll of insulating` material such as fiber orglass upon the opposite ends of which are mounted terminals Il as bymeans of a suitable sal l2. If case; y ing I0 is constructed of glass,then I2 is a suitable glass-to-metalseal. system with which the currentlimiting fuse is The ends of casing I0 are preferably closed by suitablesealing members I3 upon which vindicia giving infomation concerning the'fuse can be impressed. The fusible element illustrated at I4 is shownsimply as a long conductor such as silver or zinc or the like,

interconnecting two fuse-supporting members I5 f of conducting materialwhich in turn are electrically connected with terminals I I. Fusibleelement I4 has been illustrated as a single straight conducting memberbut it should be understood that depending upon the current rating itmay comprise one or more parallel conducting members and as will becomeapparent from the modications shown in Figs. 2 and 3, it may extend bymeans of a tortuous path from one end of the enclosing casing Ill to theother. Instead of a silver wire, a flat silver foil might also beadvantageously used with my invention as will become apparent from thefollowing description.

The arc quenching means of my invention, which I have found to greatlyincrease the current interrupting ability and current limiting action ofthe current limiting fuse of my invention, is illustrated in Fig. 1 lascomprising a pair of flat plates I6 and I 1 which are mounted inparallel planes along the axis of the fuse casing Ill. For highestcurrent limiting action it may be desirable to limit the space I8between plates I6 and Il to the diameter of the fusible element Ill sothat the plates I6 and Il are actually pressed against the fusibleelement.` It will be understood that plates vi6 and il are mounted inthis position in any-suitable manner as by being suitably clampedbetween fuse supporting members I5. The volts per centimeter, or voltagegradient, which is indicative of the current limiting action may bedecreased somewhat by increasing the sparing I8 between the plates I6and Il to a valve somewhat greater than the diameter of the fusibleelement. With this arrangement the voltage-surges produced on the systemby virtue of the current'ilimiting action are somewhatl reduced but itis believed to be more satisfactory to obtain the highest voltagegradient possible for the maximum current limiting action, by decreasingthe spacing I8 to a minimum and using a stepped wire construction,

Y for example, such as is disclosed in Lohausen Patent 2.157,907`tolimit the voltage surges produced due to the very high current limitingaction. With the arrangement described the arc The plates I6 and I"Ishould preferably be constructed from insulating material such as fusedquartz', high resistivity glass, fused alumina, opaque fused silica,lava, magnesium oxide, thorium oxide or possibly from other highicsistivity ceramics.

Although I have found that the current limiting'action of the fuse ofFig. 1 is considerably better than current limiting fuses using an inertgranular arc extinguishing ller material,- I have found that theaddition of such liller material to the disclosure described thusfar,operates as satisfactorily as without such ller and under certain casesit may be desirable to include .such filler material in the spaceremaining in casing l0.

Normally itis necessary for the fusible elements in a'current limitingfuse to be consid- -erafbly longer than the practicable Alength of thelike, around which the fusible element is spirall wound. In Figs. 2 and3 two such constructions are shown which embody `.the features of thepresent invention and permit the properties of .the fiat-plate type ofarc extinguishing structure.

Referring now to Fig. 2 I have illustrated a ceramic core I9 which mayhave an H-cros'ssection. The fusible elementI I4 is spirally woundaround the ceramic core in the well-knownimanner. Plates and 2i are thenplaced adjacent the flat surfaces of the core I9 so as to provide forsuccessive portions of' the fusible element the fiat plate geometrydescribed in connection with Fig. l. Flat plates 20 and 2i may bepressed against the fiat surfaces of the core I9 if desired, to obtainthe highest voltage gradient. It will be understood that with this typeof construction mounted in a suitable enclosing casing I0 and embeddedin a filler of an inert granular arc extinguishing materia1 such as sandindicated at-22, a sort of a composite current limiting fuse is obtainedhaving the llat plate geometry for a portion thereof and the well-knownconstruction of a fusible element embedded in sand v or the like for theother portion. The dileren- I arrangement to the composite fuseconstruction of Fig. 2 in which a ceramic core in the form of a hollowsquare 23 is provided upon which the fusible element comprising one ormore parallel conductors is spirally wound. In this case a plurality offlat plates 2t are provided which are respectively mounted adjacent theflat surfaces of the hollow square core 23. ment disclosed in Fig. 3 arelatively large portion of the fuse would employ the Ilat plateproperties described above as contrasted with Fig. 2.

From the above discussion i-t willbe obvious that I have provided a newand improved current limiting fuse which is simple in construction andwhich has a higher current interrupting ability and current limitingaction than similar fuses used heretofore. r

While I have disclosed certain particularernbodiments of my invention,it will be apparent to those skilled in the art that my invention is notlimited to the particular constructions shown but that changes andmodifications may be made without departing from the spirit and scope ofmy invention and I aim in the appended lclaims to cover all such changesand modifications.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. In a high tension fuse of the current linuting type, an enclosingcasing, an insulating support mounted in said casing,la conductorspirally wound on said insulating support, said insulating supporthaving at least one flat surface, and a flat plate of insulatingmateria1 disposed adjacent to and parallel with said flat surface toenclose portions of said spirally wound conductor between said flatsurface and said flat plate, said conductor'being adapted to dissipatethroughout a fusible section of considerable length as distinguishedfrom dissipation only at a point of reduced cross section uponsubjection of said fu.- sible section to excess current of largemagnitude, said flat surface and at plate providing an arc With thearrange-v spirally wound conductor between said fiat plates and fiatsurfaces, said conductor being adapted to dissipate throughout a.fusible section of con` siderable length as distinguished fromdissipation only ata pointof reduced cross section upon subjectionY ofsaid fusible section to excess cur-4 rent of large magnitude, said. fiatsurfaces providing an arc quenching means, capable yof pro-l ducing avery high voltage 4gradient upon operation of said fuse so as toexert ahigh current limiting action.

3. In a high tension fuse of the current limiting type comprising anenclosing casing, a longitudinally ribbed H-shaped insulating supporthaving two fiat surfaces mounted in said casing, a conductor spirallywound on -said insulating support, said conductor being adapted todis.-y sipate throughout a fusible section of considerable length asdistinguished from dissipation only at a point of reduced cross sectionupon subjection of said fusible section to excess current of largemagnitude, and aflat plate of insulating mate: rial associated with eachof the flat surfaces of said insulating support disposed adjacent' toand parallel with said flat surfaces to enclose portions of saidspirally wound conductor between said ilat surfaces and said fiatplates, said fiat surfaces and plates providing anlarc quenching meanscapable of producing a very high voltage gradient upon operation of saidfuse so as to exert a high current limiting action.

4. In a high tension fuse of the current limiting type comprising anenclosing casing, an in-I sulating support in the shape of a hollowsquare having a plurality of flat surfaces mounted in said casing, aconductor spirally wound on said insulating support, said conductorbeing adapted to dissipate throughout a fusible section of considerablelength as distinguished from dissipation only at a point of reducedcross section upon subjection of said fusible section to excess currentof vlarge magnitude, and a plurality of fiat plates of insulatingmaterial one associated with each of the fiat surfaces of saidinsulating support and disposed adjacent toand parallel with -said fiatsurfaces to enclose portions of said spiport having at least one flatsurface mounted in said casing, a conductor spirally wound on saidinsulating support said conductor being 'adaptedv y plate. f

' 6. A high tension fuse of the current limiting type `including incombination a cylindrical casing `of insulatingv material, a conductormounted within said casing and adapted to dissipate throughout a fusiblesection of considerable length as distinguished from dissipation only ata point of reduced cross-section uponv subjection of the fusible sectionto excess current of large magnitude, and arc quenching means comprisingtwo members of insulating material I mounted within said casing so as toextend longirally wound conductor between said fiatsurfaces and saidflat plates.

5. In a high tension fuse of the current limiting type. an enclosingcasing, an insulating suptudinally'thereof, each of said members havinga flat surface extending throughout substantially the whole length ofsaid casing and said members being positioned in said casing with saidfiat surfaces on opposite sides of and adjacent the conductor whereby toexert s, high current limiting action upon subjection of said conductorto excess current of large magnitude.

7. A high tension fuse of the current limiting type `including incombination a cylindrical casing of insulating material, two terminalsrespectively mounted at the ends of said casing, two fuse supportsrespectively mounted-at the ends of said casing in current conductingrelationship with the terminals at the respective ends, a conductormounted within said casing and interconnectingsaid fuse supports, saidconductor being adapted' to dissipate throughout a fusible section oi'considerable length as distinguished from dissipation only at a-point ofreduced cross-section upon subjection of the fusible section to excesscurrent of large magnitude, and arc quenchingI means comprising twomembers of fused quartz mounted within said casing so as to extendlongitudinally thereof, each of said members having a flat surfaceextending from one of said fuse supports to the other and said membersbeing positioned with said flat surfaces closely spaced and pressingagainst the fusible section of said conductor from opposite sidesthereof whereby to exert a high current limiting action upon subiectionyof the conductor to` excess current of large magnitude.

. CHAUNCEY G. SUITS.

